I have the M21-04; and have to tell you that it is big! Tactical design; and the Carson Flipper is a nice feature. Most versions come so that you can adjust the clip for left/right and tip up/down carry, I wear mine tip up w/the blade spine against the seam of my pocket.
As far as a SD knife, I can't say I'd necessarily rank it too highly. The clip is pretty tight initially (maybe if I wore it more it would change), plus the 5" handle is so long it seems to take forever to pull out and then flip open. To use the Carson flipper, you have to flick the blade open with a wrist snap - in a real SD situation it's wasted motion. I am planning to use mine as a camp knife as the wide blade seems perfect for it.
A better SD knife IMO would be a smaller folder like the smaller CRKT M21-12, a CRKT M16, a balisong, or a small fixed blade (like the now-discontinued Spyderco Ronin, the Becker Necker, an Emerson Kerambit, etc).
The Endura, being smaller, would be easier to deploy. The 4th Generation version of the blade comes in tip-up carry; which is a plus for SD applications. I don't own an Endura, but having handled the M21 and if SD is your #1 intention for the blade I can say I'd either go for the Endura of the 2 you picked, or the M21-02 over the -04.